The present invention relates to a turbulence conveyor flow meter for medical use, particularly in physiopathology, in pulmonary and cardiocirculatory medicine and in anaesthesia.
In the medical field, particularly with reference to the pulmonary function, there exists a need for exact measurements of flows within a wide range of values, from zero to hundreds of liters per minute. To carry out such measurements, two types of devices are known in the prior art, namely, laminar and turbulence flow meters.
Among laminar flow meters, the most widely used is the Fleisch pneumotachograph. It comprises an undulated thin plate spirally wound, flowing a diaphragm or laminar, which obstructs air flow. The pressure drop at the ends of the diaphragm gives a measure of the flow. Such a device, however, collects dirt and can be obstructed when the flows are unclean. Moreover, it cannot be used for high flow values, which alter the laminar motion of the flow.
To overcome these limitations, turbulence flow meters have been introduced. They consist of obstacles to the flow which produce turbulence in the flowing fluid. The pressure drop is proportional to the flow being tested according substantially to a quadratic law.
Among turbulence flow meters, the Elliot's device is known (Journal of Applied Physiology, 1975, pages 456-460) comprising a chamber, the input and output ducts of which are misaligned. The flow value is measured by the pressure drop at the two chamber ends. An advantage of such a device is its working stability and lack of sensitivity to physiologically polluted flows. On the other hand, a limitation of the device is its excessive resistance to the flow for high values of the flow (when it is higher than 120 liters per minute) and its lack of sensitivity to low values of flow (less than 3 liters per minute). In fact, for such low values, no appreciable turbulence takes place.
Flow meters provided with a resilient membrane are also known (Franetzki, Ph.D dissertation, 1975, Universitat Friedriciana Karlsruhe). In them, the resistance to the flow is inversely proportional to the flow. These flow meters can be built with a convenient material, and shaped in such a way, so as to establish a laminar relation between the pressure drop and the flow to be measured. The sensitivity to respiratory soiling (sputum) and the levity of the membranes makes these flow meters unreliable.
All of the flow meters referred to above, both turbulence and laminar flow types, possess some drawbacks in the measurement of flows. Moreover, they are not suited to the conveyance of expired flows for various reasons. This feature, though not strictly required for some tests (spirometry) becomes essential in the rebreathing technique. As known, the aim of rebreathing techniques is the analysis and/or the intervention in the respiratory and/or cardiocirculatory function for clinical or therapeutic purposes (nitrogen wash-out, oxygen rebreathing, anaesthetics inhalation).
The problem of conveying expired flows is solved at present by combining together independent flow meters and conveyors. In order to avoid measurement disturbances, the valve (conveyor) has to be placed not too near the flow meter. This causes an undesired dead space when the flow meter is placed between patient and valve; therefore, the necessity of having more flow meters to avoid the dead space, or at least a means to measure the flow in one of its passages.
Due to these limitations, some rebreathing techniques proposed in the last century (Pfueger's School, Germany, 1870-1873) and well developed by the physiologoists in the sixties (Doehring and Thews, Pfeugers Archiv. 311, 1969pages 326-341) did not find clinical application.
The object of the present invention is to overcome the limitations and drawbacks above-referred to, by providing a reliable compact device, of simple construction and easy use, in the medical field, allowing the measurement and conveyance of respiratory flows, particularly in physiopathology, in pulmonary and cardiocirculatory medicine and in anaesthesia.
The above objective is attained in accordance with the invention with a turbulence conveyance flow meter for medical use comprising a turbulence chamber with variable input and/or output areas, a transducer system for the pressure drop across the turbulence chamber and connected between two external points of the chamber, a servomechanism under control of the transducer system, acting on the flow to be measured, and a detector of the measured flow.
According to the invention, the servomechanism may be of the type acting on the input and/or output areas.
Advantageously, the flow meter may comprise a tap or valve with an intercepting element, provided with at least a passageway forming, when it is open to the flow, a turbulence chamber. Still according to the invention, the flow meter may comprise a microprocessor controlling the servomechanism, according to the signals of the detector and to the actual input/output areas of the chamber, and detecting the measured flow.
The turbulence chamber of the flow meter may be provided with at least an input duct and an output duct, for the continuous sampling of the gas to be measured and for its compensation with an equal volume of gas.
Other objects and advantages of the invention will become apparent during the course of the following description.